This invention relates to the Finnish sauna-bath or more properly the sauna, which is a form of exposure of one's body in an enclosed space to a special atmosphere of air and water at temperatures much beyond body temperature.
According to the ancient Finnish practice of sauna, small rocks are oiled in special manner, an art which is fast becoming lost, after which a wood fire is burned for hours under the rocks until they become heated to a temperature ranging between 600 F. and 1000 F. The fire is then extinguished and a measured amount of water is thrown on the rockpile.
The rocks are so arranged that most of the water rapidly reaches the bottom layers. These bottom layers are arranged to arrest the flow of water so that here the bulk of water is rapidly turned to steam. The heap of rocks is arranged to impede the free escape of the steam so that the steam, by contact with the highly heated rocks, is superheated as it rises.
The superheated expanding steam mixes with the hot dry air in and about the rockpile, creating a surge of air and steam in the sauna room. This mixture of air and superheated steam generated in a sudden surge is known by a Finnish word `loyly`; the English language has no correspondent equivalent term.
The loyly generator constructed from rocks generates proper loyly only if the temperature of the rocks is approximately 600 F. or higher. Steam which is generated at lower temperatures is not sufficiently superheated to have the same effect on a bather's body as proper loyly.
Many steam generators have been produced, mostly electricity powered, which do not generate superheated, dry steam, and therefore are not loyly generators. Nevertheless they are misleadingly marketed as `sauna heaters`. They are in effect wet steam or hot air generators, incapable of producing the loyly that characterizes a true Finnish sauna. They are not able of generating superheated, dry steam because their rockpile is very low.
Many of these units could be made to produce true loyly by pouring water on top of the rocks if: (a) their rockpile height would be increased to at least 16inches; (b) the heating time would be increased to at least 6 hours; and (c) the heating capacity would be increased to at least 15 kw. Such units, however, are impractical and uneconomical.
A rockpile height of 12 inches is a borderline --below it true loyly cannot be produced, above it the rockpile heating becomes impractical in terms of both time and power required. At around 12 inches height however, loyly can be produced by one or more of the means described in this invention with a reasonable heating capacity (6-10 kw) in a reasonable heating time (2-4 hours).
The rockpile frames for most presently marketed sauna generators are shallow round, square or rectangular containers which are filled with rocks. A heat source (a wood fire, electric elements or gas flame) is installed below the rockpile to produce hot gases which supply heat to the rocks.
If water is poured on top of these rockpiles, most of the water turns to steam on the surface of the rocks at the very top of the rockpile. This action is undesirable because it cools the top rocks so that the generated steam is too wet to form true sauna loyly and if the rockpile is not high enough, the remaining water sinks through the entire rockpile without ever becoming steam.
In all sauna generators in which water is poured or thrown onto a rockpile, rising steam touches the hot rocks and expands forming a tornado-like cone. This is an inherent property of steam rising through heated rocks. The volume of the cone increases exponentially with increases in the height of the cone. In a 4-inch rockpile, the volume of the cone may be noticeably less than 20 cubic inches, while in a 12-inch rockpile the volume increases to approximately 500 cubic inches, and in 16-inch rockpile to well over 1000 cubic inches. The volume of this steam cone is a direct measure of the generator's ability to generate loyly.
If water could be spread out near the bottom of the rockpile over a horizontal area the size of the top of the cone, the volume of hot rocks touched by the rising steam may be increased several times over the volume of rocks touched by a cone-like formation. The capacity to generate superheated, dry steam, therefore, would be increased correspondingly if the steam would be expanded to a larger space below the rockpile before it rises up through the hot rockpile.